In a typical wireless communication system, wireless service providers may operate radio access networks (RANs), each RAN including a number of base stations that each radiate to provide coverage in which wireless client devices (WCDs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped devices, can operate. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a WCD within coverage of the RAN may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other WCDs served by the base station.
In practice, physical base station equipment in such a RAN may be configured to provide multiple specific coverage areas, or “cells,” differentiated from each other by direction, carrier frequency, or the like. For simplicity in this description, however, each cell may be considered to correspond with a respective base station and each base station may be considered to correspond with a respective cell. Thus, an arrangement where physical base station equipment provides multiple cells could be considered to effectively include multiple base stations, each providing a respective one of those cells.
Further, a RAN may operate in accordance with a particular air interface protocol or “radio access technology,” with communications from the base stations to WCDs defining a downlink or forward link and communications from the WCDs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA (e.g., Long Term Evolution (LTE) or Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1xRTT and 1xEV-DO), Global System for Mobile Communications (GSM), and Wi-Fi, among others. Each protocol may define its own procedures for managing communications with WCDs.
In accordance with the air interface protocol, each cell may operate on one or more carrier frequencies. Further, each cell may define a number of channels or specific resources for carrying signals and information between the base station and WCDs. For instance, on the downlink channel, certain resources may be used to carry a pilot or reference signal that WCDs may detect and measure as a basis to evaluate coverage, and other resources may be used to carry paging messages and other such control messages from the base station to WCDs. And on the uplink channel, certain resources may be used to carry registration requests and other control messages from WCDs to the base station. In addition, certain resources on the uplink and downlink may be set aside to carry bearer traffic (e.g., user communications) between the base station and the WCDs.
When a WCD first powers on or enters into coverage of a RAN, the WCD may search for a cell with the strongest coverage and may then register with the RAN by transmitting a registration message to the base station that provides that cell. The WCD may then operate in an idle mode or a connected (or active) mode in that cell. In the idle mode, the WCD may periodically monitor transmissions from the base station in search of any paging messages directed to the WCD and, upon detecting such a paging message or at the initiation of the WCD, may transmit a control message to the base station to facilitate transition to the connected mode. In the connected mode, the WCD may then engage in bearer communication, such as voice calls and/or data sessions, on air interface resources assigned by the base station.
Further, in both the idle mode and connected mode, the WCD may regularly monitor the strength of its serving cell and the strength of any other available cells, to help ensure that the WCD operates in the best available coverage. If the WCD detects that the strength of its serving cell is threshold low and/or that the strength of another cell is sufficiently high, and/or for other reasons, the WCD may then hand over to operate in the other cell. In the connected mode, this handover process would involve signaling between the WCD and the RAN to facilitate transfer of the WCD's connection to the new cell. Whereas, in the idle mode, the handover process may involve the WCD simply transitioning to idle in the new cell, beginning to monitor transmissions from the base station in the new cell in search of any paging messages directed to the WCD.